In many applications of polymeric materials, particularly in the use of polymeric composites in structural applications, it is necessary to have accurate knowledge of any existing flaw's size, shape and location. The use of polymeric composite materials has been increasing at a tremendous rate. The reasons for this trend are that these materials have very attractive strength-to-weight ratio and dimensional stability. To fully utilize the potential of these materials, nondestructive techniques for characterization of flaws are becoming key elements in the manufacturing processes and in-service maintenance of these materials. Moreover, to minimize inspection cost and to maximize inspection reliability and accuracy, an automated, on-line system is highly desirable.
Existing techniques such as ultrasonics, x-radiography, thermal imaging, microwave techniques, holographic techniques, and acoustic emisions are not suitable for on-line measurements. Notwithstanding advances in numerous NDE (nondestructive evaluation) techniques, notably in the use of ultrasonics, a fully automated, on-line inspection system is still not available for composite and polymeric materials. One of the major obstacles for developing an automated, on-line system is that interpretation of the measurement output in order to determine the nature and the characteristics of the defects is still a difficult problem. Moreover, most techniques can only be used at fairly low speed. For instance, ultrasonics, the work horse for inspection of composite materials, suffers from most of these problems, in addition to other questions on reproducibility of measurements and convenience in use. Other available techniques such as x-ray radiography, dye-penetrant, eddy current, thermal imaging techniques, corona and microwave methods, holographic techniques, and acoustic emissions, etc. suffer from one problem or another. For example, the ultrasonic technique requires a coupling medium between the ultrasonic transducer and the object-under-test for acoustic impedance matching; radiation hazard is a problem in using x-ray; microwave techniques can only be used for fairly large defects due to the long wavelength; and holographic technique using laser optics has stringent requirements on mechanical stability. Hence, there is a need for a technique which is sensitive and quantitative for detecting and characterizing defects. At the same time, this technique should be simple and low cost, thus lending itself to cost-effective, on-line inspection.